A standard line-type foliage trimmer has a housing that is rotated about an axis and that contains a spool of a line that acts as a cutting element. The free end of the line extends through an eye from the housing and the spool is normally fixed in the housing, so that when the housing is rotated at high speed the free line end extends radially from the housing and whips about, cutting any light foliage it comes in contact with.
This line, which is typically nylon monofilament, is subject to enormous wear and miscellaneous mechanical action, so that the end breaks off periodically. Thus it is necessary to pay out some more line, to which end some sort of mechanism must be provided that normally holds the spool against rotation while allowing some limited rotation to unwind a new section of line when necessary.
French Pat. No. 2,429,550 and U.S. Pat. No. 4,347,666 describe a system wherein a latch element is provided which is movable between an inner position blocking rotation of the spool in the housing and an outer position permitting such rotation. A spring urges the element into the inner position and centrifugal force is oppositely effective on it. Thus when the line becomes quite short its air resistance will decrease and the rotation speed of the housing will increase. This increases the centrifugal force so that the latch element moves out and allows the spool to pay out some more line. Since the spring force is substantially constant, such a device will therefore only work for a given rotation or drive speed. Thus, if, for instance, the apparatus is old and cannot get up to maximum speed because the drive motor and bearings are worn, the arrangement does not work. In addition such arrangements work in a jerky manner, suddenly releasing and rearresting the spool, so that they are subject to considerable shock and wear and are therefore not reliable.
U.S. Pat. Nos. 4,290,200, 4,236,312, and 4,244,103 use a gripper arrangement that directly engages the strand, pinching it against the housing immediately adjacent the eye. A centrifugally operated spring-biased clamping pawl is normally used in these arrangements, making them speed-sensitive as described above.
Other systems are known which pay out a section of line each time the head is bumped on the ground. These arrangements are disadvantageous in that the decision to pay out more line is left up to the operator. In addition it is fairly common in use to bump the bottom of the head and accidentally send out more line, wasting the cutting strand.
In U.S. Pat. Nos. 4,352,243 and 4,366,621 latching arrangements are shown which do not respond to speed. These arrangements are extremely complex and have short service lives.